W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
The dedicated channel (DCH) for downlink and uplink and the downlink shared channel (DSCH) have been defined in Release 99 and Release 4. In the following years, the developers recognized that for providing multimedia services—or data services in general—high speed asymmetric access had to be implemented. In Release 5 the high-speed downlink packet access (HSDPA) was introduced. The new high-speed downlink shared channel (HS-DSCH) provides downlink high-speed access to the user from the UMTS Radio Access Network (RAN) to the communication terminals, called user equipments in the UMTS specifications.
Hybrid ARQ Schemes
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which are combined with Forward Error Correction (FEC), called Hybrid ARQ. If Cyclic Redundancy Check (CRC) detects an error, the receiver requests the transmitter to send additional bits or a new data packet. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit will be encoded before transmission. Depending on the bits that are retransmitted three various types of ARQ may be defined.
In HARQ Type I the erroneous data packets received, also called PDUs (Packet Data Unit) are discarded and new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU. Using HARQ Type II the erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Finally, HARQ Type III is almost the same packet retransmission scheme as Type II and only differs in that every retransmitted PDU is self-decodable. This implies that the PDU is decodable without the combination with previous PDUs. In case some PDUs are heavily damaged such that almost no information is reusable self decodable packets can be advantageously used.
When employing chase-combining the retransmission packets carry identical symbols. In this case the multiple received packets are combined either by a symbol-by-symbol or by a bit-by-bit basis (see D. Chase: “Code combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets”, IEEE Transactions on Communications, Col. COM-33, pages 385 to 393, May 1985). These combined values are stored in the soft buffers of respective HARQ processes.
Packet Scheduling
Packet scheduling may be a radio resource management algorithm used for allocating transmission opportunities and transmission formats to the users admitted to a shared medium. Scheduling may be used in packet based mobile radio networks in combination with adaptive modulation and coding to maximize throughput/capacity by e.g. allocating transmission opportunities to the users in favorable channel conditions. The packet data service in UMTS may be applicable for the interactive and background traffic classes, though it may also be used for streaming services. Traffic belonging to the interactive and background classes is treated as non real time (NRT) traffic and is controlled by the packet scheduler. The packet scheduling methodologies can be characterized by:                Scheduling period/frequency: The period over which users are scheduled ahead in time.        Serve order: The order in which users are served, e.g. random order (round robin) or according to channel quality (C/I or throughput based).        Allocation method: The criterion for allocating resources, e.g. same data amount or same power/code/time resources for all queued users per allocation interval.        
The packet scheduler for uplink is distributed between Radio Network Controller (RNC) and user equipment in 3GPP UMTS R99/R4/R5. On the uplink, the air interface resource to be shared by various users is the total received power at a Node B, and consequently the task of the scheduler is to allocate the power among the user equipment(s). In current UMTS R99/R4/R5 specifications the RNC controls the maximum rate/power a user equipment is allowed to transmit during uplink transmission by allocating a set of various transport formats (modulation scheme, code rate, etc.) to each user equipment.
The establishment and reconfiguration of such a TFCS (transport format combination set) may be accomplished using Radio Resource Control (RRC) messaging between RNC and user equipment. The user equipment is allowed to autonomously choose among the allocated transport format combinations based on its own status e.g. available power and buffer status. In current UMTS R99/R4/R5 specifications there is no control on time imposed on the uplink user equipment transmissions. The scheduler may e.g. operate on transmission time interval basis.
UMTS Architecture
The high level R99/4/5 architecture of Universal Mobile Telecommunication System UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, incorporated herein by reference). The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the user equipments. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called Iu connection with the Core Network (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supports the Serving RNS (S-RNS) 301 by providing radio resources as shown in FIG. 3. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used.
Enhanced Uplink Dedicated Channel (E-DCH)
Uplink enhancements for Dedicated Transport Channels (DTCH) are currently studied by the 3GPP Technical Specification Group RAN (see 3GPP TR 25.896: “Feasibility Study for Enhanced Uplink for UTRA FDD (Release 6)”, incorporated herein by reference). Since the use of IP-based services become more important, there is an increasing demand to improve the coverage and throughput of the RAN as well as to reduce the delay of the uplink dedicated transport channels. Streaming, interactive and background services could benefit from this enhanced uplink.
One enhancement is the usage of adaptive modulation and coding schemes (AMC) in connection with Node B controlled scheduling, thus an enhancement of the Uu interface. In the existing R99/R4/R5 system the uplink maximum data rate control resides in the RNC. By relocating the scheduler in the Node B the latency introduced due to signaling on the interface between RNC and Node B may be reduced and thus the scheduler may be able to respond faster to temporal changes in the uplink load. This may reduce the overall latency in communications of the user equipment with the RAN. Therefore Node B controlled scheduling is capable of better controlling the uplink interference and smoothing the noise rise variance by allocating higher data rates quickly when the uplink load decreases and respectively by restricting the uplink data rates when the uplink load increases. The coverage and cell throughput may be improved by a better control of the uplink interference.
Another technique, which may be considered to reduce the delay on the uplink, is introducing a shorter TTI (Transmission Time Interval) length for the E-DCH compared to other transport channels. A transmission time interval length of 2 ms is currently investigated for use on the E-DCH, while a transmission time interval of 10 ms is commonly used on the other channels. Hybrid ARQ, which was one of the key technologies in HSDPA, is also considered for the enhanced uplink dedicated channel. The Hybrid ARQ protocol between a Node B and a user equipment allows for rapid retransmissions of erroneously received data units, and may thus reduce the number of RLC (Radio Link Control) retransmissions and the associated delays. This may improve the quality of service experienced by the end user.
To support enhancements described above, a new MAC sub-layer is introduced which will be called MAC-eu in the following (see 3GPP TSG RAN WG1, meeting #31, Tdoc R01-030284, “Scheduled and Autonomous Mode Operation for the Enhanced Uplink”). The entities of this new sub-layer, which will be described in more detail in the following sections, may be located in user equipment and Node B. On user equipment side, the MAC-eu performs the new task of multiplexing upper layer data (e.g. MAC-d) data into the new enhanced transport channels and operating HARQ protocol transmitting entities.
E-DCH MAC Architecture at the User Equipment
FIG. 4 shows the exemplary overall E-DCH MAC architecture on user equipment side. A new MAC functional entity, the MAC-eu 503, is added to the MAC architecture of ReI/99/4/5. The E-DCH MAC architecture includes RLC and higher layer entities 501, a MAC-d entity 502. the new functional entity MAC-eu 503, and the Physical Layer 504. The MAC-eu 503 entity is depicted in more detail in FIG. 5.
There are M various data flows (MAC-d) carrying data packets to be transmitted from user equipment to Node B. These data flows can have different QoS (Quality of Service), e.g. delay and error requirements, and may require different configurations of HARQ instances. Therefore the data packets can be stored in different Priority Queues. The set of HARQ transmitting and receiving entities, located in user equipment and Node B respectively will be referred to as HARQ process. The scheduler will consider QoS parameters in allocating HARQ processes to different priority queues. MAC-eu entity receives scheduling information from Node B (network side) via Layer 1 signaling.
E-DCH MAC Architecture at the UTRAN
In soft handover operation the MAC-eu entities in the E-DCH MAC Architecture at the UTRAN side may be distributed across Node B (MAC-eub) and S-RNC (MAC-eur). The scheduler in Node B chooses the active users and performs rate control by determining and signaling a commanded rate, suggested rate or TFC (Transport Format Combination) threshold that limits the active user (UE) to a subset of the TCFS (Transport Format Combination Set) allowed for transmission.
Every MAC-eu entity corresponds to a user (UE). In FIG. 6 the Node B MAC-eu architecture is depicted in more detail. It can be noted that each HARQ Receiver entity is assigned certain amount or area of the soft buffer memory for combining the bits of the packets from outstanding retransmissions. Once a packet is received successfully, it is forwarded to the reordering buffer providing the in-sequence delivery to upper layer. According to the depicted implementation, the reordering buffer resides in S-RNC during soft handover (see 3GPP TSG RAN WG 1, meeting #31: “HARQ Structure”, Tdoc R1-030247, incorporated herein by reference). In FIG. 7 the S-RNC MAC-eu architecture which comprises the reordering buffer of the corresponding user (UE) is shown. The number of reordering buffers is equal to the number of data flows in the corresponding MAC-eu entity on user equipment side. Data and control in-formation is sent from all Node Bs within Active Set to S-RNC during soft handover.
It should be noted that the required soft buffer size depends on the used HARQ scheme, e.g. an HARQ scheme using incremental redundancy (IR) requires more soft buffer than one with chase combining (CC).
E-DCH Signaling
E-DCH associated control signaling required for the operation of a particular scheme consists of uplink and downlink signaling. The signaling depends on uplink enhancements being considered.
In order to enable Node B controlled scheduling (e.g. Node B controlled time and rate scheduling), user equipment has to send some request message on the uplink for transmitting data to the Node B. The request message may contain status information of a user equipment e.g. buffer status, power status, channel quality estimate. The request message is in the following referred to as Scheduling Information (SI). Based on this information a Node B can estimate the noise rise and schedule the UE. With a grant message sent in the downlink from the Node B to the UE, the Node B assigns the UE the TFCS with maximum data rate and the time interval, the UE is allowed to send. The grant message is in the following referred to as Scheduling Assignment (SA).
In the uplink user equipment has to signal Node B with a rate indicator message information that is necessary to decode the transmitted packets correctly, e.g. transport block size (TBS), modulation and coding scheme (MCS) level, etc. Furthermore, in case HARQ is used, the user equipment has to signal HARQ related control information (e.g. Hybrid ARQ process number, HARQ sequence number referred to as New Data Indicator (NDI) for UMTS ReI. 5, Redundancy version (RV), Rate matching parameters etc.)
After reception and decoding of transmitted packets on enhanced uplink dedicated channel (E-DCH) the Node B has to inform the user equipment if transmission was successful by respectively sending ACK/NAK in the downlink.
Mobility Management within R99/4/5 UTRAN
In this section some frequently used terms will be briefly defined and some procedures connected to mobility management will be outlined (see 3GPP TR 21.905: “Vocabulary for 3GPP Specifications”, incorporated herein by reference).
A radio link may be a logical association between single UE and a single UTRAN access point. Its physical realization comprises radio bearer transmissions.
A handover may be defined as transfer of a user's connection from one radio bearer to another. In contrast, during “soft handover” (SHO) radio links are established and abandoned such that the UE always keeps at least one radio link to the UTRAN. Soft handover is specific for networks employing Code Division Multiple Access (CDMA) technology. Handover execution is commonly controlled by S-RNC in mobile radio network.
The “active set” comprises a set of radio links simultaneously involved in a specific communication service between UE and radio network, e.g. during soft handover, the UE's active set comprises all radio links to the RAN's Node Bs serving the UE.
An Active set update procedure modifies the active set of the communication between UE and UTRAN. The procedure comprises three functions: radio link addition, radio link removal and combined radio link addition and removal. The maximum number of simultaneous radio links is set to eight. New radio links may be added to the active set once the pilot signal strengths of respective base stations exceed certain threshold relative to the pilot signal of the strongest member within active set. A radio link may be removed from the active set once the pilot signal strength of the respective base station exceeds certain threshold relative to the strongest member of the active set. The threshold for radio link addition is typically chosen to be higher than that for the radio link deletion.
Hence, addition and removal events form a hysteresis with respect to pilot signal strengths. Pilot signal measurements are reported to the network (S-RNC) from the UE by means of RRC signaling. Before sending measurement results, some filtering my be performed to average out the fast fading. A typical filtering duration may be about 200 ms and the duration contributes to handover delay. Based on measurement results, S-RNC may decide to trigger the execution of one of the functions of active set update procedure.
E-DCH—Node B Controlled Scheduling
Node B controlled scheduling is one of the technical features for E-DCH which is foreseen to enable more efficient use of the uplink power resource in order to provide a higher cell throughput in the uplink and to increase the coverage. The term “Node B controlled scheduling” denotes the possibility for the Node B to control, within the limits set by the RNC, the set of TFCs from which the UE may choose a suitable TFC. The set of TFCs from which the UE may choose autonomously a TFC is in the following referred to as “Node B controlled TFC subset”. “Node B controlled TFC subset” is a subset of the TFCS configured by RNC as seen in FIG. 8. The UE selects a suitable TFC from the “Node B controlled TFC subset” employing the ReI5 TFC selection algorithm. Any TFC in the “Node B controlled TFC subset” might be selected by the UE, provided there is sufficient power margin, sufficient data available and TFC is not in the blocked state. Two fundamental approaches to scheduling UE transmission for the E-DCH exist. The scheduling schemes can all be viewed as management of the TFC selection in the UE and mainly differs in how the Node B can influence this process and the associated signaling requirements.
Node B Controlled Rate Scheduling
The principle of this scheduling approach is to allow Node B to control and restrict the transport format combination selection of the user equipment by fast TFCS restriction control. A Node B may expand/reduce the “Node B controlled subset”, which user equipment can choose autonomously on suitable transport format combination from, by Layer-1 signaling. In Node B controlled rate scheduling all uplink transmissions may occur in parallel but at a rate low enough such that the noise rise threshold at the Node B is not exceeded. Hence, transmissions from different user equipments may overlap in time. With Rate scheduling a Node B can only restrict the uplink TFCS but does not have any control of the time when UEs are transmitting data on the E-DCH. Due to Node B being unaware of the number of UEs transmitting at the same time no precise control of the uplink noise rise in the cell may be possible (see 3GPP TR 25.896: “Feasibility study for Enhanced Uplink for UTRA FDD (Release 6)”, version 1.0.0, incorporated herein by reference).
Two new Layer-1 messages are introduced in order to enable the transport format combination control by Layer-1 signaling between the Node B and the user equipment. A Rate Request (RR) may be sent in the uplink by the user equipment to the Node B. With the RR the user equipment can request the Node B to expand/reduce the “Node controlled TFC Subset” by one step. Further, a Rate Grant (RG) may be sent in the downlink by the Node B to the user equipment. Using the RG, the Node B may change the “Node B controlled TFC Subset”, e.g. by sending up/down commands. The new “Node B controlled TFC Subset” is valid until the next time it is updated.
Node B Controlled Rate and Time Scheduling
The basic principle of Node B controlled time and rate scheduling is to allow (theoretically only) a subset of the user equipments to transmit at a given time, such that the desired total noise rise at the Node B is not exceeded. Instead of sending up/down commands to expand/reduce the “Node B controlled TFC Subset” by one step, a Node B may update the transport format combination subset to any allowed value through explicit signaling, e.g. by sending a TFCS indicator (which could be a pointer).
Furthermore, a Node B may set the start time and the validity period a user equipment is allowed to transmit. Updates of the “Node B controlled TFC Subsets” for different user equipments may be coordinated by the scheduler in order to avoid transmissions from multiple user equipments overlapping in time to the extent possible. In the uplink of CDMA systems, simultaneous transmissions always interfere with each other. Therefore by controlling the number of user equipments, transmitting simultaneously data on the E-DCH, Node B may have more precise control of the uplink interference level in the cell. The Node B scheduler may decide which user equipments are allowed to transmit and the corresponding TFCS indicator on a per transmission time interval (TTI) basis based on, for example, buffer status of the user equipment, power status of the user equipment and available interference Rise over Thermal (RoT) margin at the Node B.
Two new Layer-1 messages are introduced in order to support Node B controlled time and rate scheduling. A Scheduling Information Update (SI) may be sent in the uplink by the user equipment to the Node B. If user equipment finds a need for sending scheduling request to Node B (for example new data occurs in user equipment buffer), a user equipment may transmit required scheduling information. With this scheduling information the user equipment provides Node B information on its status, for example its buffer occupancy and available transmit power.
A Scheduling assignment (SA) may be transmitted in the downlink from a Node B to a user equipment. Upon receiving the scheduling request the Node B may schedule a user equipment based on the scheduling information (SI) and parameters like available RoT margin at the Node B. In the Scheduling Assignment (SA) the Node B may signal the TFCS indicator and subsequent transmission start time and validity period to be used by the user equipment.
Node B controlled time and rate scheduling provides a more precise RoT control compared to the rate-only controlled scheduling as already mentioned before. However this more precise control of the interference at this Node B is obtained at the cost of more signaling overhead and scheduling delay (scheduling request and scheduling assignment messages) compared to rate control scheduling.
In FIG. 9 a general scheduling procedure with Node B controlled time and rate scheduling is shown. When a user equipment wants to be scheduled for transmission of data on E-DCH it first sends a scheduling request to Node B. Tprop denotes here the propagation time on the air interface. The contents of this scheduling request are information (scheduling information) for example buffer status and power status of the user equipment. Upon receiving that scheduling request, the Node B may process the obtained information and determine the scheduling assignment. The scheduling will require the processing time Tschedule.
The scheduling assignment, which comprises the TFCS indicator and the corresponding transmission start time and validity period, may be then transmitted in the downlink to the user equipment. After receiving the scheduling assignment the user equipment will start transmission on E-DCH in the assigned transmission time interval.
The use of either rate scheduling or time and rate scheduling may be restricted by the available power as the E-DCH will have to co-exist with a mix of other transmissions by the user equipments in the uplink. The co-existence of the different scheduling modes may provide flexibility in serving different traffic types. For example, traffic with small amount of data and/or higher priority such as TCP ACK/NACK may be sent using only a rate control mode with autonomous transmissions compared to using time and rate-control scheduling. The former would involve lower latency and lower signaling overhead.
E-DCH—Hybrid ARQ
Node B controlled Hybrid ARQ may allow rapid retransmissions of erroneously received data packets. Fast retransmissions between a user equipment and a Node B may reduce the number of higher layer retransmissions and the associated delays, thus the quality perceived by the end user may be improved.
A protocol structure with multiple stop-and-wait (SAW) Hybrid ARQ processes can be used for E-DCH, similar to the scheme employed for the downlink HS-DSCH in HSDPA, but with appropriate modifications motivated by the differences between uplink and downlink (see 3GPP TR 25.896).
An N-channel SAW scheme consists of N parallel HARQ process, each process works as a stop-and-wait retransmission protocols, which corresponds to a selective repeat ARQ (SR) with window size 1. It is assumed that user equipment can only transmit data on a single HARQ process each transmission time interval.
In FIG. 10 an example N-channel SAW protocol with N=3 HARQ processes is illustrated. A user equipment is transmitting data packet 1 on E-DCH on the uplink to the Node B. The transmission is carried out on the first HARQ process. After propagation delay of the air interface Tprop the Node B receives the packet and starts demodulating and decoding. Depending on whether the decoding was successful an ACK/NACK is sent in the downlink to the user equipment.
In this example Node B sends an ACK after TNBprocess, which denotes the time required for decoding and processing the received packet in Node B, to the user equipment. Based on the feedback on the downlink the user equipment decides whether it resends the data packet or transmits a new data packet. The processing time available for the user equipment between receiving the ACKnowledgement and transmitting the next transmission time interval in the same HARQ process is denoted TUEprocess.
In the example user equipment transmits data packet 4 upon receiving the ACK. The round trip time (RTT) denotes the time between transmission of a data packet in the uplink and sending a retransmission of that packet or a new data packet upon receiving the ACK/NACK feedback for that packet. To avoid idle periods due to lack of available HARQ processes, it is necessary that the number N of HARQ processes matches to the HARQ round trip time (RTT).
Considering known and unknown transmission timing, it may be distinguished between synchronous and asynchronous data transmission. A retransmission protocol with asynchronous data transmission uses an explicit signaling to identify a data block or the HARQ process, whereas in a protocol with synchronous data transmission, a data block or HARQ process is identified based on the point of time a data block is received.
A UE may for example have to signal the HARQ process number explicitly in a protocol with asynchronous data transmission in order to ensure correct soft combining of data packets in case of a retransmission. The advantage of a HARQ retransmission protocol with asynchronous data transmission is the flexibility, which is given to the system. The Node B scheduler may for example assign UEs a time period and HARQ processes for the transmission of data on the E-DCH based on the interference situation in the cell and further parameters like priority or QoS parameters of the corresponding E-DCH service.
A retransmission protocol with asynchronous HARQ feedback information uses sequence numbers (SN) or other explicit identification of the feedback messages whereas protocols with synchronous HARQ feedback information identifies the feedback messages based on the time when they are received, as for example in HSDPA. Feedback may be sent on the HS-DPCCH after a certain time instant upon having received the HS-DSCH (see 3GPP TR 25.848: “Physical Layer Aspects of High Speed Downlink Packet Access”, version 5.0.0, incorporated herein by reference).
As mentioned before a retransmission protocol with asynchronous data transmission enables the Node B more scheduling flexibility. The scheduling assignment can for example be based on the scheduling information sent from UE and the interference situation in the cell. The different scheduling approaches considering retransmissions have to be taken into account, in order to enable further control of the uplink interference by the Node B scheduler.
A retransmission protocol with asynchronous uplink but synchronous retransmissions may be one approach, which allows the scheduler more control on the noise rise in the cell. The transmission of new data packets on E-DCH is sent in an asynchronous manner in order to keep the advantage of scheduling flexibility, though the retransmissions are sent after a predefined time instant upon having received the NACK. The advantages of a retransmission protocol with synchronous retransmissions may also depend on the scheduling mode used.
In the rate controlled scheduling mode Node B is only controlling the TFCS and the UE can choose among an appropriate TFC for the uplink transmissions. Node B has no control on the UEs transmission time. There is also no restriction on the retransmission timing for the UE. Employing a retransmission protocol with synchronous retransmissions Node B exactly knows when the retransmissions are sent by UE, hence it can reserve uplink resources, which enables Node B a more precise control on the uplink interference in the cell.
In a time and rate controlled scheduling mode Node B schedules the initial—as well as the retransmissions sent on the E-DCH. In case retransmissions are sent in a synchronous manner, Node B doesn't need to schedule the retransmissions anymore, which reduces the signaling overhead and the processing time for the scheduler in Node B significantly. The retransmission is sent Tsync after having received the NACK. UE doesn't have to monitor the grant channel for a scheduling assignment (SA) for the retransmission.
Due to transmitting retransmissions a fixed time period after receiving the NACK (Tsync), there are delay benefits on UE side. In case retransmissions are also scheduled, Node B could assign transmission resources to other UEs instead of scheduling the pending retransmissions.
In multi-access communication systems, techniques to reduce mutual interference between multiple users may be utilized, in order to increase the capacity. By means of for example power control techniques the transmission power of each user may be limited to a certain value that is necessary to achieve a desired quality of service. This approach may ensure that each user transmits only the power necessary, thereby making only the smallest possible contribution to the total noise seen by other users.
In this respect the interference caused by retransmissions may be an important issue. In order to improve the capacity and hence in order to increase the coverage and throughput of a communication system, it may be desirable to keep the uplink interference caused by retransmissions as small as possible. Especially in interference critical situations, where a lot of transmissions are most likely received in error, it may be desirable that the corresponding retransmissions do not increase the interference level in the cell significantly.
For example in a UMTS environment using a HARQ retransmission protocol with synchronous retransmissions, it may be desirable that the interference in the cell is not significantly increased due to a huge number of retransmissions, since a Node B may not have control on the retransmission timing.
Furthermore employing a HARQ retransmission scheme may increase the data transmission efficiency, i.e. throughput and system performance, in a mobile communication system. The power used for retransmissions may for example be reduced by using the information received from the previously erroneously data packet to decode the retransmitted data packet.
More specifically, the soft decisions from the previous corrupted data packet may be soft combined with the retransmitted data packet. Therefore the energy per bit (EbNt) required for a successful decoding of the data packet may be reduced for the retransmission.
However in a conventional HARQ scheme the transmitting entity upon receiving a retransmission request has no knowledge of the reception quality of the previously incorrectly received data packet and hence does not know what retransmission power level is required for a successful decoding after soft combining.
For example when utilizing a HARQ retransmission scheme with Incremental Redundancy (IR) the transmitting entity does not know how much additional information (redundancy) is required for a successful decoding. When the transmission power for the retransmission data packet is too low or the amount of redundancy is not sufficient, the decoding will most probably fail. Hence, the delay is increased due to further required retransmissions. On the other hand if the retransmission power is more than required for a successful decoding resources are wasted, which could have been allocated for other initial transmissions.
US 2003/0235160 A1 describes an adaptive gain adjustment for the retransmission power of retransmission. The document proposes a retransmission protocol, where initial transmissions are transmitted at a first power level and retransmission data packets are transmitted with reduced power. Power control by means of the gain for retransmissions is adapted by adjusting the traffic-pilot ratio in order to optimise the performance. The adjustment of the traffic-pilot ratio is based on whether or not the previous retransmission data packet was successfully decoded by the base station.